Hello Artjom,

Good day. Thank you for reaching out to us.

We would like to ask for the turn ratio of your transformer to evaluate the exact value of the Zener. For the time being, you may refer to the device design guide on page 8 under 'The Zener diode voltage.' It states the Zener diode to be selected is approximately 6 V above the bias winding voltage.

Primary winding 100 N

Auxiliary winding 10 N

Secondary winding 20 N

Hi Artjom,

Thank you for your response. We suggest a Zener diode of 25V.

Thank you for the response. There is one thing I don't understand. At al load of approximately 0.6 Amps, the auxiliary voltage is around 38V. According to the datasheet, the Zener diode voltage should be about 6V above the bias winding. For this reason, I would have chosen a 44V Zener diode. However, if the Zener diode has a voltage of 25V, it will conduct, and the current Isd will increase. Or am I misunderstanding something? 

Thanks

Hi Artjom,

Thank you for your prompt response. You are correct; I apologize for the confusion.

From our understanding, since the voltage of the auxiliary is 12V at no load, we conclude that your output is 24V as per your turn ratio.

Our suggested calculation for the zener diode at the primary OVP is equal to the overvoltage of the output minus the BP/M voltage. And the overvoltage is 125% of the output voltage. So, our initial computation is as follows: (24*1.25)-VSHUNT = 25 V 

Now, considering the bias voltage at full load, the calculation must be the one presented on the attached.

Hence, your understanding of using 44V is correct.

I hope this works for you.
 

Thank you very much for your response. You have helped me a lot. I have one more question: I selected the resistor R8 with 22 kΩ as specified in the datasheet. This works well so far. However, PI Expert suggests a value of about 47 kΩ. Which value is better? In idle mode, the current  Isd (BP/M) is about 800 µA, and from 0.1 Amps, the Isd current is constant at about 1.23 mA at maximum load. How high should the minimum  Isd  current be? Is the idle current of 800 µA too low?

Thanks 

Hi Artjom,

Good day. Could you please provide us with the PI Expert you've generated so that we can review it further?

Hello, here is just an example of the auxiliary voltage. When idling, the auxiliary voltage is 12V and at maximum load 38V. I'm not sure which value to choose for R4 22 or 47?

Thanks

Hi Artjom,

Thank you for your response.

According to the IC bias current considerations in the device design guide, your bias supply must be at least 850 uA during normal operation. Since your measurement is around 1.23 mA, this set-up should work. We can only consider the bias during normal operation; hence, the 800 uA in idle operation is acceptable.

And in regard to which RBias (R4) to use, whether 22 k or 47 k, since we are now using 22 k, we can continue using it if the design passes the no-load requirement; otherwise, we can try using the suggested value of the PI Expert. The PI Expert suggests values that can be used as a base value for RBias and can be adjusted depending on the test performance and the load requirement of the design.

Thank you for your response. I have one last question: What is the maximum allowable auxiliary voltage at BP/M? I assume that 38V is acceptable?

Thank you for you response. Yo have been helpful. 

I have now measured directly at the BP/M. At idle, the voltage is 5.89V, and as soon as more than 0.1 amps flow, the voltage is constant at 6.23V, even at maximum load. According to the datasheet, the voltage should not exceed 6.15 V. Did I make a mistake somewhere? 

Thanks.

Hi Artjom,

Thank you for your response.

The 38V at full load for the auxiliary winding is actually too high. Possibly the coupling of the windings is not good. And we can consider decreasing the turns for the auxiliary.

But in regard to the BP/M at a voltage constant of 6.23 V at maximum load is fine, at normal operation we should consider the BP/M Pin Shunt Voltage which ranges from 6 V to 6.7 V value to be measured in BP/M. There is an internal shunt regulator clamping the BP/M pin at around 6.4 V when current is provided from an external source, which is the auxiliary winding.

Hello,

I probably didn’t express myself correctly. I meant the auxiliary voltage that is measured at an auxiliary winding. How high or what should be the maximum value? I have attached an example of how it was measured. My question is whether the voltage at partial load is acceptable. The datasheet recommends selecting the auxiliary voltage between 15-30 V. In my design, the auxiliary voltage at full load is approximately 38 V.

Thanks

Hi Artjom,

The maximum value we can consider from the design guide, as you've stated, is 30 volts; hence, we suggest decreasing the number of turns in your auxiliary.

And in addition to measuring the VBP/M to be just less than the VSHUNT, the voltage at partial load is not acceptable. We suggest configuring the RBias, R4, and decreasing it so the measure of the VBP/M is at the minimum load in the range of VSHUNT. 

With 10 turns, the auxiliary voltage is about 13 V at no load. This is a good value for standby power. If I choose 8 turns instead of 10, the voltage at no load is about 10.5 V, which results in higher standby power. Therefore, I don’t understand the datasheet specification of 15 V to 30 V. On one hand, there should be a minimum voltage at no load to achieve lower standby power, and on the other hand, the voltage should not exceed 30 V. It is clear that the auxiliary voltage increases with the load, isn’t it?

Hello again, now I understand. I changed the RBias resistor from 22 kΩ 6.8 kΩ . And the auxiliary voltage is now about 27 V at maximum load. I didn’t realize that the auxiliary voltage could vary with the RBias. Thanks for your effort.